In recent years, for coexistence of cost reduction and higher efficiency of solar cells, thin film solar cells consuming only a small amount of raw materials attract attention, and wholehearted development is now being performed. Especially, expected is, as a method for enabling cost reduction, a method for forming an excellent semiconductor layer on an inexpensive transparent base material, such as glass, using a low-temperature process.
When such a thin film solar cell is manufactured as a thin film solar cell having a large area enabling generation of electric power with a high output at a high voltage, generally used is a method wherein a plurality of thin film solar cells formed on a substrate with a large area connected in series are not used, but in order to obtain excellent yield, a thin film solar cell formed on the substrate with a large area is divided into a plurality of cells, and the cells are then connected in series to realize integration. In a thin film solar cell, using a glass plate as a substrate, especially having a configuration wherein light is entered from a glass substrate side, generally after sequential formation of semiconductor layers on a glass substrate, isolation grooves are applied by processing the transparent electrode in a shape of strips with a specified width, using a laser scribe method, in order to reduce a loss by a resistance of a transparent electrode layer on the glass substrate, and then each cell is connected in series in a direction perpendicular to a longitudinal direction of the strips to realize integration.
FIG. 2 is a conceptual plan view of such an integrated type thin film solar cell.
FIG. 3 is a sectional view of a structure of an area surrounded with an ellipse 2A in FIG. 2.
FIG. 4 is a more detailed sectional 3.
view of a stacked structure of an area surrounded with an ellipse 3A in Figure
Generally in manufacture of an integrated type thin film solar cell 6 as shown in FIGS. 2 to 4, a glass substrate 11 is used as a transparent insulating substrate 11. On the glass substrate 11, a SnO2 film having a thickness of 700 nm, for example, is formed by a heat CVD method as a transparent electrode layer 12. The transparent electrode layer 12 is isolated into transparent electrodes with strip-shape having a width W of approximately 10 mm by forming a transparent electrode layer isolation groove 62 having a width of about 100 micrometers with a laser scribe method. Residues after scribed are removed by ultrasonic cleaning using organic solvents or water. As cleaning methods, methods of removing residues using pressure sensitive adhesives, injection gases, etc. may also be used.
Furthermore, after formation of one or more of amorphous units 2 or crystalline photoelectric conversion units 3, these units are divided within a plane thereof into a plurality of areas with strip-shape with connection groove 63. Since, this connection groove 63 is used for electrical connection of the transparent electrode layer 12 and the back face electrode layer 4, between mutually adjacent cells, partial remaining of residue of the scribe do not occur problems, and therefore ultrasonic cleaning may be omitted. Then, formation of a back face electrode layer 4 electrically connects the back face electrode layer 4 to the transparent electrode layer 12 formed in a shape of a strip as mentioned above via the connection groove 63.
The back face electrode layer 4 is formed in a manner that it is patterned by a same laser scribe method as in the one or more crystalline photoelectric conversion units 3 or amorphous unit 2, then is locally blown away with one or more crystalline photoelectric conversion unit 3 or amorphous units 2 to form a plurality of back electrode isolation grooves 64, and then ultrasonic cleaning is given. This method enables formation of solar cells 61 having a shape of a plurality of strips, and realizes mutual electric series connection through the connection groove 63 of those cells. Finally, a back face side of the thin film solar cell is protected by additional sealing resin (not shown).
In thin film solar cells, although they may have thinner photoelectric conversion layers as compared with layers in solar cells using conventional bulk type monocrystalline or polycrystalline silicon, they have a problem that light absorption of a whole thin film will be limited by a film thickness. Then, in order to more effectively utilize light that entered into a photoelectric conversion unit including a photoelectric conversion layer, proposed are designing ideas for increasing an amount of light absorption within the photoelectric conversion layer. In the technique, unevenness is given to a surface of transparent conducting layer or a metal layer in contact with the photoelectric conversion unit (texturizing) to scatter the light in an interface of the surface, and then to enter the light into the photoelectric conversion unit. This technique is referred to as “light trapping”, and is important elemental technology for practical use of thin film solar cells having high photoelectric conversion efficiency.
Amorphous silicon cells as an example of thin film solar cells are usually formed on a transparent base material, such as glass, and use tin oxide (SnO2) film having surface unevenness thereon as a transparent electrode layer. Surface unevenness of this transparent electrode layer effectively contributes to light trapping within the photoelectric conversion layer. However, a glass base material having SnO2 film formed by a thermal chemical vapor deposition method (thermal CVD method) as a transparent electrode layer having a surface unevenness effective in light trapping has a problem of necessity of a high temperature process at approximately 550 to 650 degree C. in order to form the transparent electrode layer, leading to a problem of high manufacturing cost. High film-forming temperatures lead to a problem of disabling use of inexpensive base materials, such as glass and plastic film, after solidification. Exposure to high temperature processes of tempered glass reduces tempered effect, and disables use as a base material of the tempered glass, therefore use of thicker glass is needed in order to secure a strength of the glass base material in application in large area solar cells, resulting in heavier products.
Moreover, the SnO2 film has low plasma-proof property, and therefore reduction of SnO2 film may be induced under environment of deposition of a photoelectric conversion layer with a greater electron density using hydrogen. A reduced SnO2 film blackens, therefore blackened portions of the transparent electrode layer absorbs incident light, and decreases an amount of the transmitted light into the photoelectric conversion layer, causing decrease in conversion efficiency.
Additionally, amorphous silicon cells have lower initial photoelectric conversion efficiency as compared with monocrystal or polycrystal solar cells, and moreover have a problem of decrease in conversion efficiency due to photodegradation phenomenon. Therefore, crystalline silicon thin film solar cells using crystalline silicon like thin film polycrystalline silicon or micro crystallite silicon as a photoelectric conversion layer is expected and investigated as a device enabling coexistence of low cost and high efficiency. Reasons are that crystalline silicon thin film solar cells may be formed at lower temperatures by a plasma CVD method in a same manner as in formation of amorphous silicon, and furthermore may be formed almost without photodegradation phenomenon. Although amorphous silicon photoelectric conversion layers can perform photoelectric conversion of light in wavelengths of approximately 800 nm in a longer wavelength side, crystalline silicon photoelectric conversion layers can perform photoelectric conversion of light in longer wavelengths up to approximately 1200 nm. However, it needs a larger plasma density than a density in deposition conditions used for formation of amorphous silicon, and therefore in using SnO2 film for transparent electrodes, greater improvement in conversion efficiency has been difficult.
Terms of “crystalline” and “micro crystallite” as used in the present specification also include a state where amorphous material is partially included.
On the other hand, zinc oxide (ZnO) has advantageously higher plasma resistance than that of SnO2 or indium oxide tin (ITO) widely used as materials of transparent electrode layers, and furthermore it has lower price, and therefore, it is preferable as a transparent electrode layer material for thin film solar cells.
(Prior Art 1)
For example, Japanese Patent Laid-Open No. 2000-252501 official report discloses a formation method of ZnO film wherein a thin film having unevenness may be formed by a low pressure thermal CVD method (also referred to as MOCVD method) at temperatures not more than 200 degrees C. Since the method is a low-temperature process at temperatures not more than 200 degrees C., it can achieve lower cost compared with that of the high pressure thermal CVD. The method can use inexpensive base materials, such as glass and plastics films after solidification. Use of tempered glass may provide large area solar cell glass base materials having smaller thickness of approximately ⅔ than that in conventional method, resulting in lighter substrates. Furthermore, the low pressure thermal CVD method enables film formation faster by an order of magnitude than that in sputtering technique, and higher effective utilization efficiency of raw materials, and therefore the method is preferable for thin film solar cells also in respect of manufacturing cost.
(Prior Art 2)
On the other hand, Japanese Patent Laid-Open No. 2003-243676 official report indicates a technique of forming transparent electrode layers, wherein in order to give unevenness to a substrate for thin film solar cells, a foundation layer having unevenness is formed on a surface of a glass base material, and then a transparent electrode layer is formed thereon, instead of forming unevenness on the transparent electrode layer itself. In the method, formed on a glass base material is a foundation layer having unevenness comprising insulating micro-particles having an average particle diameter of 0.1 to 1.0 micrometers and binder, subsequently on which a transparent electrode layer is deposited. Thereby, since fine unevenness is formed on the glass substrate by the micro-particles, the transparent electrode layer in particular itself does not necessarily have formed unevenness thereon.